In a body of automobiles such as motorcycles, cars and the like, used is a suspension integrated with a shock absorber for reducing the vibration of the body caused by road surface roughness, the shaking thereof which occurs in quick acceleration or sudden braking, and the like. The structure of the shock absorber is based on a cylindrical structure that utilizes the resistance of oil to flow. Specifically, as a shock absorber, used is one produced by forming small holes through a hydraulic piston. In accordance with the upward and downward movement of the piston, oil passes through the holes, and the resistance during the movement is proportional to the speed of the piston. In the slide part between the cylinder and the piston rod, placed is a bush that prevents the mutual abration and also acts as a guide, and the part is sealed up with a rubber sealant for preventing oil leakage. Various types of shock absorbers are known, such as double pipe-type ones, gas-filled ones, etc.
Recently, vibration of running automobiles has become suppressed owing to improvement in road conditions, etc., and in many cases, the expansion and contraction movement of shock absorbers has become reduced. When the friction between the rubber sealant and the piston rod is large in such a case where the expansion and contraction movement of a shock absorber is small, the vibration would run through the car to worsen the riding comfort. In particular, in the case of a motorcycle, the vibration transmitted to the handle is felt greatly, thereby often detracting from the riding comfort.
As a lubricating oil composition for a shock absorber, widely used is one containing a zinc dialkyldithiophosphate (PTLs 1 and 2).
However, nothing is discussed in PTLs 1 and 2, relating to the way to reduce the friction between the rubber sealant and the piston rod in the case where the expansion and contraction movement of a shock absorber is small, in which, therefore, the riding comfort could not be improved in the case where the expansion and contraction movement of a shock absorber is small.
For reducing the friction coefficient of a rubber sealant, it is known that a phosphorus-containing additive is effective (PTL 3).
However, the phosphorus-containing additive has a problem in that the thermal stability thereof is poor. In particular, in the case of motorcycles, the shock absorber is not sealed up with nitrogen, and therefore the lubricating oil composition therein is often oxidized at high temperatures, and the importance of thermal stability is great.
PTL 1: JP 2009-13380 A
PTL 2: JP 5-255682 A
PTL 3: JP 5-255683 A